Cellulose is used in the production of a number of products well known in the art. One product is a cellulose food casing. These generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.
A non-reinforced type of cellulose casing commonly is used in the manufacture of skinless hot dogs. These cellulose food casings generally consist of a tubular film of pure regenerated cellulose having a wall thickness ranging from about 0.025 mm to about 0.038 mm and in diameters of from about 14 to 50 mm. For some purposes, larger casings are used where the tubular wall thickness is up to 0.076 mm and the diameter is up to about 203 mm.
Another cellulose product is cellulose film. For many years a transparent cellulose film commonly known as cellophane was the film of choice for use as a wrapping and packaging material. Both casing and cellophane are most commonly produced by the well known "viscose process".
In this process, a natural cellulose, such as wood pulp or cotton linters, first is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged, and treated with carbon disulfide to form cellulose xanthate. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution, or "viscose", is ripened, filtered, deaerated, and extruded.
For use as a food casing, the viscose is extruded as a tube through an annular die and about a self-centering mandrel into acidic coagulation and regenerating baths containing salts and sulfuric acid. For films, the extrusion can be as a sheet or as a tube that is later slit to form a sheet of film.
In the acidic baths the cellulose xanthate, is converted back to cellulose. The acid bath decomposes the cellulose xanthate in a chemical reaction with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state, the cellulose product is first run through a series of rinse water dip tanks to remove by-products formed during regeneration.
During regeneration, the chemical reaction liberates sulfur product and gases such as hydrogen sulfide, carbon disulfide, and carbon dioxide through both surfaces of the gel. These gases are noxious and toxic, so their containment and recovery imposes a considerable burden on the manufacturing process. Moreover, when extruded as a tube, gases generated at the internal surface of the extruded gel tube can accumulate within the tubing and consequently present special problems. The pressure build up of gases accumulating within gel tubing causes undesirable diameter variations and therefor variations in film thickness and width. To prevent this, the gel tubing is punctured periodically to vent the accumulated gases. This puncturing process, involving procedures to puncture, vent, and then seal the punctured gel tube, results in an undesirable interruption of the manufacturing process. Also, gases which evolve may become entrapped within the structure of the gel, causing bubbles that weaken the resulting casing or film.
The gel product, to some extent, retains low residual levels of the sulfur compounds produced during regeneration. While care is taken to remove all residual sulfur compounds by washing the gel tube or film, the final product may still contain trace amounts of these compounds. The gel product then is treated with a glycerine humectant and dried to about 15% moisture based on total casing weight. For purposes of reference, a cellulose which is derivatized and then regenerated back to cellulose by a chemical reaction in the viscose process is sometime referred hereinafter to "viscose cellulose".
When formed as a tube, the viscose cellulose gel product is inflated during the drying process and stretched longitudinally to provide a degree of transverse and machine direction orientation to the dried cellulose tube.
It is known in the food casing art that a degree of both machine direction ("MD") and transverse direction ("TD") orientation, particularly to provide wet strength, is required for acceptable use of the cellulose tube as a food casing. For example, orientation decreases the extensibility of the casing but increases tensile strength. However, to function as a food casing some degree of extensibility must be retained. Thus, orientation must be accomplished so as to balance the desirable properties of both extensibility and tensile strength in both the MD and TD directions.
One method of producing a stronger viscose cellulose casing is seen in U.S. Pat. No. 2,999,757. Here, an extruded viscose tube is fully regenerated and some degree of orientation is provided by maintaining the tube in an inflated condition as it passes through a dryer. The inflation is sufficient to impart a 35% to 55% circumferential stretch and the pull through the drier is sufficient to impart a 2% to 8% machine direction stretch during drying. The result is a food casing having a wall thickness (dry) of 0.94 mils (0.037 mm) which when rewet retains substantially all of the stretch imparted during drying.
This patent further suggests that 55% TDO is about the maximum possible for viscose cellulose tubing. According to the '757 Patent, when the viscose cellulose is stretched transversely more than 55%, many operating problems are encountered. First, it is difficult to transversely stretch cellulose casing during drying beyond about 55% as the casing is more likely to break in the drier when the TDO is excessive. Further, as casing is stretched over about 55%, it rapidly loses desirable physical properties. The resulting casing has little or no residual stretch upon rewetting and will tend to break excessively when subjected to the rigors of stuffing and processing the stuffed casing.
In addition to placing an upper limit of about 55% on the transverse stretch, the '757 Patent also indicates that the film wall thickness is critical. In the operative examples of the '757 Patent, the wall thickness of the casing (when dry) preferably is about one mil.
An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance, as in the viscose process. U.S. Pat. No. 2,179,181 discloses the dissolution of natural cellulose by a tertiary amine N-oxide to produce solutions of relatively low solids content, for example, 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine N-oxide. Later patents provide for increasing the amount of cellulose in the solution. The cellulose in the resulting solution is nonderivatized prior to dissolution.
For purposes of this specification, "nonderivatized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding.
Such solutions, when extruded into a nonsolvent, cause the dissolved cellulose to regenerate by precipitation. For purposes of this invention "nonsolvent" means a liquid which is not a cellulose solvent. This alternate cellulose production method has been used primarily to produce filaments and fibers rather than films.
U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide ("NMMO") as the tertiary amine N-oxide cellulose solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the '939 Patent.
Canadian patent No. 1,171,615 discloses a dialysis membrane formed of nonderivatized cellulose by extrusion through a spinneret having a slot-width of 180 mm and a gap adjustment of 0.6 mm.
German Patent 42 19 658 C2 suggests manufacture of film strips having a thickness (dry) of about 0.012 mm which is formed by extrusion of a nonderivatized cellulose through a flat sheet die having a die length of 4 cm. Various die widths were used from 0.005 cm to 0.010 cm to make the film strips.
Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose and consequently, it eliminates problems associated with chemical reactions such as the generation of toxic and noxious gases and sulfur compounds. For purposes of reference, a cellulose which is dissolved by NMMO and then regenerated back to cellulose by contacting the solution with a nonsolvent is sometimes referred to hereinafter as "non derivatized cellulose" or "NMMO cellulose".
Even with this advantage, to applicants' knowledge, and prior to the disclosure in the parent application (now U.S. Pat. No. 5,277,857), solutions of NMMO and cellulose have been used primarily to manufacture fibers and filaments and not in the commercial manufacture of cellulose films or food casings. This may be due in part to the fact that the solution exhibits thermoplastic behavior with a melting point of about 65.degree. C., so it is normally solid at the temperature used in the extrusion of viscose.
It is speculated that another reason why this solution has not been commercially used in manufacture of tubular food casings or films is that at 65.degree. C. it has a viscosity significantly higher than the viscosity of the viscose heretofore used in the production of cellulose food casings. In particular, a solution of cellulose and NMMO may have a molecular weight of about 80,000 to 150,000 and a viscosity in the range of about 1,000,000 to 3,500,000 centipoise. The high molecular weight and viscosity is because the dissolution of the cellulose does not affect the degree of polymerization. Viscose, for manufacture of frankfurter casing (wherein the degree of polymerization is affected by the derivatization process), has a molecular weight in the range of about 95,000 to 115,000 and a viscosity of only 5,000 to 30,000 centipoise.
From a cellulose article manufacturing process standpoint, these differences are important because after dissolution the process steps are dependent on whether cellulose has entered into a covalent bond with the solubilizing reagent, i.e., has been derivatized. This is so in the case of the well-known and commercially practiced viscose process. When a cellulose derivative is processed into the shaped article, the derivative such as viscose is first partially coagulated in the extrusion bath and then subsequently hydrolyzed back to cellulose, i.e., cellulose is regenerated. During this hydrolysis and while the derivative is still in a "plastic" state, the reforming cellulose crystallites can be stretched and oriented to give desirable commercial properties such as high tensile strength or burst strength. However, a disadvantage of this general approach is that since a cellulose derivative has been hydrolyzed, additional byproducts are formed. This significantly complicates cellulose recovery.
By contrast, when there is a direct cellulose dissolution such as by a solvent concentration of NMMO and water, orienting the cellulose molecules during the reorganization of the cellulose article is more difficult because there is no covalent bond to break. So reorganization is essentially a physical dilution or decomplexation. However recovery is less complex and, at least in the cellulose/NMMO/H.sub.2 O system, is commercially feasible.
U.S. Pat. No. 4,246,221 and East German Patent No. DE 218 121, teach that solutions of cellulose, NMMO and water may be extruded through a spinneret and longitudinally pulled through a 12 inch long air gap into a precipitating bath to form very small diameter solid fibers which have a tensile strength, as measured in grams per denier, greater than comparable rayon fibers regenerated from viscose.
It will be appreciated by those skilled in the art, that manufacture of cellulose fibers and filaments by extrusion through orifices only 2-4 mils in diameter, or extrusion through slot dies, is nonanalogous to the manufacture, by extrusion, of large diameter tubular films having a minimum inside diameter of 0.5 inches (12.7 mm) or more and a wall thickness of 0.0015 inch (0.038) or less.
In U.S. Pat. No. 5,277,857 ("'857 Patent") there is disclosed a method and apparatus for manufacturing a large diameter (at least 14.5 mm) tubular film, suitable for use as a food casing, from a cellulose solution, in particular, a solution of cellulose, an amine oxide cellulose solvent (NMMO) and water (hereinafter sometimes referred to as "dope"). As disclosed in this patent, the solution, solid at room temperature, is melted and extruded through an air gap and into a nonsolvent liquid such as a water bath.
In the water bath, the nonderivatized cellulose regenerates by precipitation. The resulting gel tube can be treated with water, a polyhydric alcohol such as glycerine, or other water soluble softening agent such as a polyalkylene oxide or a polyalkylene glycol prior to drying.
While tubular films of nonderivatized cellulose were successfully made using the teachings of the '857 Patent, it was found that the tubular films were of limited use. This is because the transverse tensile strength needed improvement, as did the balance between the MD and TD tensile strengths.
To some extent, the TD tensile strength of tubular films prepared by the teachings of the '857 Patent were improved by increasing the length of the air gap. This improvement is disclosed in a application Ser. No. 08/179,418 filed Jan. 10, 1994 now U.S. Pat. No. 5,451,364 which is incorporated herein by reference. In brief, it was found that if the extruded tube was passed through an air gap length of about 6 to 12 inches, the resulting tube, dried under low inflation pressure, produced a tubular film 0.80 to 1.20 mils thick wherein the ratio of MD:TD tensile strength of the rewet film was about 2 or less and the TD tensile strength was at least about 2.0 lbs/in.multidot.mil (about 1670 to 2500 psi). However, even these values, and particularly the TD tensile strength, needs improvement to compete favorably with a conventional cellulose food casing made using the viscose process.
Accordingly, one object of this invention is to provide a method of forming a seamless tube of nonderivatized cellulose having a relatively high TD tensile strength and an improved balance of the MD:TD tensile strength.
As noted above, another cellulose product is cellulose film as may be used for wrapping various articles such as candy, cigars and in other clear overwrap packaging markets. For purposes of making cellophane sheets, the viscose generally is cast or extruded through a slot die. Drying usually is accomplished by supporting the gel cellulose on heated drums. Slot casting of sheets allows some degree of orientation in the machine direction by controlling the speeds at which the cast film is pulled forward from the slot die and through the regenerator baths.
As the sheet dries, it shrinks. If transverse direction shrinkage is not restrained, the thickness of the film is increased and the width decreases. Restraint of transverse shrinkage is minimized by tentering and this provides the film with some degree of transverse orientation. Typically, a cellophane cast as a sheet will, after drying, be on the order of a mil thick (0.025 mm) and have an unbalanced MD and TD orientation. For example, it may have an MD tensile strength of 18,000 psi (124.13 MPa) and a TD tensile strength of 8,000 psi (55.17 MPa).
Due in part to this unbalanced nature and in part to the rise in use of bioriented polypropylene, the use of cellophane has declined. Polypropylene is a relatively inexpensive thermoplastic material that is melted and extruded as a tube. While still soft, the tube is expanded both diametrically and longitudinally to provide a thin film which has a high degree of biorientation. For example, biaxially oriented polypropylene ("BOPP") films are made in thicknesses of 0.8 mils (0.02 mm) or less having MD and TD tensile strengths both above 20,000 psi (137.93 Mpa). Moreover, percent elongation of BOPP films at break can be above 100% in MD and above 40% in TD, making them attractive replacements for cellophane. As a result, cellophane has been replaced by BOPP for the packaging of many items such as cigarettes, small candies, cigars, and in other clear overwrap packaging markets.
The teachings of U.S. Pat. No. 2,949,757 discussed above do not solve the orientation limitations of cellophane in view of the perceived limitation of 55% as the upper limit of transverse stretch. For example, the teaching of this patent is embodied in a tubular film for food casings sold by Viskase Corporation under its trademark NOJAX. These films have an MD tensile strength (wet) on the order of about 4.18 lbs/in.multidot.mil and a TD tensile strength on the order of about 3.15 lb/in.multidot.mil. While this is a relatively balanced structure, the TD tensile strength is well below that of BOPP. Accordingly, even if this tube were slit to form flat sheets, its dry tensile properties and thickness should not compete favorably with BOPP.
Inflating beyond the upper limit of 55% suggested by the '757 Patent, may improve TD tensile properties but is difficult to accomplish on a commercial scale and compromises other film properties as noted above.
Attempts have been made to improve the biorientation of cellophane and therefore the balance of MD/TD tensile properties. For example, U.S. Pat. No. 3,280,234 discloses a method for producing a cellulose film with similar MD and TD tensile properties. The method involves producing viscose and extruding it as a tube directly into an acid bath without passing through air. In the acid bath the pressure within the extruded tube is slightly elevated, immediately expanding the extruded tube from 1.5 to 3 times the extruded diameter. Also, the tube is pulled through the bath at a rate that stretches it in the machine direction from 1.5 to 3 times its extruded dimension. The cellulose, which is partly regenerated in the acid bath, then is removed from the acid bath and expanded an additional 20% to 50%. The expanded tube then is slit into flat sheets. These sheets pass through additional baths to complete the regeneration and then the film is dried. The result is a viscose cellulose film having a dry thickness of from one mil (0.0254 mm) down to about 0.6 mil (0.152 mm) with MD and TD tensile strengths of up to 21,200 psi (146.2 Mpa). Evidently this process has not produced a commercially viable product because such cellophane film apparently has not regained market share from BOPP.
Even though cellulose is relatively inexpensive, the resulting viscose cellulose film on a per pound basis is still relatively expensive as compared to BOPP. This is due to the specific gravity differences between cellulose (typically 1.4 to 1.6) and BOPP (typically 0.9 to 1.0). A further disadvantage of cellophane production utilizing the viscose process is the liberation of sulfur compounds during regeneration as described above. The evolution of these gaseous by-products causes bubbles to form within the cellulose film as it regenerating and weakens it so that breakage occurs when stretching to orient the film. This is particularly the case as the film thickness decreases so that currently, cellophane film is usually at least about one mil thick.
Thus, given the low cost of cellulose, it should be appreciated that an improvement in the biorientation of a cellulose film which has a dry thickness of less than one mil, could offset somewhat the commercial advantages enjoyed by BOPP.
Accordingly, another object of the present invention is to provide an improved method for forming a cellophane film with balanced MD and TD orientation, and with a dry strength that is comparable to polypropylene film.